Means for controlling distortion in a cathode ray tube



Feb. H, 19% E. GOSTYN 3,427,497

MEANS FOR CONTROLLING DISTORTION IN A CATHODE RAY TUBE Filed July 7,1965 Sheet of 2 mvzmon ERA/6 57' 66575 ATTORNEY Feh H, 1969 E. GOSTYN3,427,497 v MEANS FOR CONTROLLING DISTORTION IN A CATHODE RAY TUBE FiledJuly 7, 1965 Shet 2 of 2 C I; 35 F L; +24 I627 C 3 KL INVENTOR 519M6257' 60: r/A/ ATTORNEY United States Patent 3,427,497 MEANS FORCONTRGLLING DISTORTION IN A CATll-IODE RAY TUBE Ernest Gostyn,Longmeadow, Mass, assignor to General Instrument orporation, acorporation of Delaware Filed .lt'uly 7, 1965, Set. No. 470,024 US. Cl.31527 17 Claims Int. Ci. Htllj 29/70; Htllf 21/00 ABSTRACT OF THEDISCLGSURE Magnetic apparatus for controlling distortion in a cathoderay tube and capable of having the magnitude and distribution of thecorrecting effect baried, a permanent magnet being rotatable relative tothe fixed magnetic structure to vary the magnitude of the correctingeffect and being shiftable along the magnetic structure for varying thedistribution of the correcting effect.

The present invention relates to means for controlling or modifyingdistortion in cathode ray tube displays, particularly distortion of thepincushion type. It is particularly well adapted for use in conjunctionwith color display tubes.

The so-called pincushion type distortion of cathode ray tube displayshas long been recognized. In black-andwhite displays this type ofdistortion is to considerable extent corrected through the use ofpermanent magnets which are so shaped and so fixed in position relativeto the cathode ray tube as to produce an appropriate magnetic biasingeffect on the cathode ray beam. However, in the case of color displaytubes which are based on use of the shadow mask or similar principlessuch fixed correcting magnets cannot be used.

One approach which has been adopted in connection with pincushiondistortion correction in color displays involves the modulation orvariation of one of the sweep currents in such a fashion as to producethe desired re sults. It has further been suggested in the prior artthat this modulation be accomplished in an electromagnetic manner, usinga combination of magnetic and electrical circuitry which works on theprinciple of magnetic saturability. In general, adequate nominalcorrection is produced by this means, but only for one particular designof display system, and it is difficult if not impossible for a givendevice to be readily adjustable or modifiable so as to compensate forthe individual perculiarities of a series of display tubes as they comefrom a production line, or to make correction adjustments from time totime in order to compensate for changes in the operating characteristicsof the cathode ray tube which occur with use. Thus it has beendifficult, on a production line, to produce a large number of colordisplays all of which have the same degree of excellence insofar asabsence of pincushion distortion is concerned, and it has beencorrespondingly difficult to prevent an increase in pincushiondistortion in a color display tube as it is used.

The prime object of the present invention is to a highly effectivemodulation of the sweep current of a display tube in order to minimizeor eliminate pincushion distortion, the magnitude of the compensatingcorrection being readily variable, and the relative magnitudes of thecorrections at the opposite extremes of the display likewise beingvariable. Thus a single system design can be used with display tubes ofdifferent characteristics, or with display tubes nominally having thesame characteristics but in which individual such tubes depart somewhatfrom their nominal characteristics.

In accordance with the present invention the electromagnetic modulationof the sweep current is accomplished in electromagnetic fashion, as inthe prior art, but the 3,427,497 Patented Feb. 11, 1969 ice system is soconstructed and arranged as to provide for adjustable variation in themodulating effect, thereby to produce the desired variations inmagnitude or location of the distortion-correcting effects. Morespecifically, the modulating structure comprises a magnetic circuithaving a plurality of legs with which coils are associated, the legsbeing connected by a magnetizable arm in such a fashion that a currentthrough one winding will effect or modulate the current in anotherwinding. A permanent magnet is associated with the magnetic circuit inorder to provide a desired magnetic bias, the location and magnitude ofthe bias determining, at least in part, the modulation effect produced.The permanent biasing magnet is formed separately from the main magneticcircuit, and is mounted so as to be movable relative to a portion of thecore with which it is magnetically associated, thereby to vary itsbiasing effect. In order to vary the maximum amplitude of the modulatingeffect produced, the biasing magnet is rotatably mounted relative to themain magnetic circuit so that the direction of magnetization of thepermanent magnet may be shifted between positions parallel to andperpendicular to that arm of the magnetic circuit with which the biasingmagnet is associated. In addition, the biasing magnet is mounted fortranslation along the arm of the magnetic circuit with which it isassociated, so that the magnitude of its biasing effect can be varied asbetween different portions of the magnetic circuit. In this way thepincushion correction can be made greater at one end of the display thanat the other end thereof, and the absolute magnitude of the correctionproduced can be independently adjusted and controlled.

It is preferred that the biasing magnet be mounted on a carriage whichis itself movably mounted on the basic magnetic circuit, the carriagebeing shiftable in position in order to produce the desired distributionof magnetic biasing effect in the different portions thereof, [and meansare provided for reliably retaining the carriage in its adjustedposition. The biasing magnet may itself be rotatably mounted on thecarriage, thereby to permit variation of the absolute magnitude of themagnetic biasing force produced independently of the distributionthereof among the various parts of the basic magnetic circuit.Additional structural arrangements may be utilized to enhance andintensify the effect of the biasing magnet on the basic magnetic circuitand thus produce a desired degree of adjustment sensitivity.

To the accomplishment of the above, and to such other objects as mayhereinafter appear, the present invention relates to a device forcontrolling distortion in a cathode ray tube, as defined in thefollowing claims and as described in this specification, taken togetherwith the accompanying drawings, in which:

FIG. 1 is atop plan view of one structural embodiment of the presentinvention;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a side elevational view thereof;

FIG. 4 is a schematic representation illustrating the geometricrequirements for correction of pincushion effect;

FIG. 5 is a graphical representation of a typical sweep current cyclewithout correction in accordance with the present invention;

FIG. 6 is a view similar to FIG. 5 but showing the sweep current withthe correction applied; and

FIG. 7 is an electromagnetic circuit diagram showing the use of thedevice of FIGS. 1-3 for producing sweep current corrections of the typeshown in FIG. 6.

Pincus-hion distortion is a type of distortion in which the outlines ofa nominally rectangular display depart from rectangularity in a concavefashion, the resultant outline having the shape of a conventionalpincushion in which the corners are spaced outwardly from the midpointof the sides. FIG. 4 discloses, in more or less schematic form, such adistortion. For simplicity of explanation, the entire display isrepresented by five horizontal lines, a-e, which represent horizontalsweeps which are repeated in order cyclically for each vertical scan orsweep. The central line is straight, but is shorter than the otherlines. The lines I) and d are curved, and the lines a and e are curvedstill more, the length of the lines b and d being intermediate betweenthat of the line 0 and those of the lines a and e. It will be seen fromFIG. 4 that deviation from the rectangular pattern occurs at the top andbottom and at the left and right sides. Either type of deviation ordistortion can be corrected in accordance with the present invention. Inthe description to follow correction will be 7 applied to the top andbottom type of pincushion distortion.

From an examination of FIG. 4 it will be seen that the top and bottomtype of pincushion distortion can be counteracted if the vertical sweepcurrent is varied in accordance with the horizontal sweep, with thatvariation in vertical sweep current changing as the horizontal sweepmoves from line a to line b and on to line 2. Line a will be convertedto a straight line (thus eliminating the top and bottom pincushioneffect) if the vertical sweep current is reduced as the horizontal beamdeflection approaches the left and right sides. The same is true of lineb, except that the amount of reduction in the vertical sweep currentwill be less than that of line a. For line 0 the vertical sweep currentshould remain constant. The correction for line d is similar to that forline b, but in the opposite direction, and the correction for line e issimilar to that for line a, but in the opposite direction.

A typical vertical deflection current wave, in idealized form, isillustrated graphically in FIG. 5. The graph portion 2 represents thechange in vertical deflection current during a single display cycle,while the graph portion 4 indicates the more rapid change of thevertical sweep current as it resets to its initial condition, ready forthe next cycle. It will be understood that during the time that thegraph portion 2 goes from maximum to minimum the horizontal sweep occursa multiplicity of times, thereby to form the horizontal sweep lines a-e(plus as many other horizontal sweep lines as are necessary to produce adisplay of desired quality. In a typical TV display the vertical sweepoccurs at the rate of 60 c. p.s., while the horizontal rate occurs at15,750 c.p.s.).

Typical points On the curve 2 corresponding to the horizontal sweeps a-eare identified in FIGS. 5 and 6 with corresponding letters. FIG. 6represents the idealized graph of FIG. 5 with pincushion distortioncorrecting modulations applied thereto. As shown in FIG. 6, thecorrecting modulation applied at point a is of the same magnitude as,but in the opposite direction from, the modulation correction applied atpoint e, the modulation corrections at points b and d are similarlyrelated to one another but have a magnitude less than that applied atpoints a and e respectively, and no modulation correction at all isapplied at point 0.

To produce the modulations shown in FIG. 6, the electromagnetic circuitshown in FIG. 7 is employed. It comprises a ferromagnetic core generallydesignated 6 and having legs 8, 10 and 12 connected at their lower endsby arm 14 and at their upper ends by arm 16. The cathode ray tubevertical deflection windings 18 and 20 are energized by a vertical sweepcurrent source 22 having an output of the type shown. The ends of thedeflection windings 18 and 20 are connected by wires 24 in series with awinding 26 on the intermediate leg 10 of the magnetic core 6. Thehorizontal deflection windings 28 and 30 of the cathode ray tube areconnected in series with a horizontal sweep current source 32, having anoutput of the character shown, and they are further connected by wires34 in series with one another and with windings 36 and 38 on the outerlegs 8 and 12 respectively of the magnetic core 6. A permanent magnet40, magnetically polarized as indicated, is magnetically operativelyassociated with the upper arm 16 of the magnetic core 6. The biasingmagnetic field produced by the magnet 40 is represented by the brokenline arrows 42. Those arrows, it will be noted, extend in oppositedirections through the legs 8 and 12, and with the magnet 40 centrallypositioned along the arm, none of that biasing field passes through theleg 10. The magnetic field produced in the leg 10 by the current in thewinding 26 is represented by the dot-dash arrows 44. These arrows 44, itwill be noted, extend in the same direction through the legs 8 and 12.During the time that current is flowing in the winding 26 in a givendirection the current through the windings 36 and 38 will alternate,because the horizontal scan is accomplished many times for each verticalscanl The magnetic field produced in the core 6 by reason of thehorizontal deflection current in the windings 36 and 38 is representedby the solid line arrows 46, which are double-headed to indicate thattheir direction alternates. The windings 26 correspond to the gatewinding of a magnetic amplifier, while the windings 36 and 38 contitutethe control windings. The horizontal deflection current variationsproduce variations in the magnetic fields indicated by the solid linearrows 46. These, in conjunction with the biasing magnetic field 42 andthe excitation status of the magnetic fields in the legs 8 and 12,control the permeability of the leg 10 and thus affect the reactance ofthe Winding 26. That reactance will vary in step with the horizontalsweep current, and as the reactance varies, the magnitude of the currentflowing therethrough, and hence the magnitude of the current flowingthrough the vertical deflection windings 18 and 2.0, will also vary.

The modulation effect on the Winding 26 will be determined not only bythat portion of the field 46 which passes through the leg 10, but alsoby the instantaneous current in the winding 26 itself. Thus with thevertical sweep current declining from its maximum value toward zero(represented by the point c on graph 2) there will be a constantlydeclining inductive drop in the winding 26, and the modulation thereofby the current in the windings 36 and 38 will therefore also decline.When the vertical deflection beam is half-way down (when the current inwinding 26 is zero) there will be no modulation .eflect. As the verticalsweep current rnOVes down below zero the current through the winding 26will increase in the opposite direction, and hence the modulation effectthereon will increase, but in the opposite direction. Thus modulationsof the types schematically indicated in FIG. 6 are produced.

Further analysis of the operation of the magnetic system of FIG. 7reveals that the amplitude of the correcting modulation produced for anygiven value of vertical deflection current (other than zero) will varyin accordance with the magnitude of the biasing magnetic fieldrepresented by the arrows 42. Analysis also reveals that with thevertical sweep current in one direction one leg 8 or 12 of the core 6predominates over the other in effecting modulation control, and whenthe vertical deflection current is in the other direction it is theother of the legs 8 or 12 which predominates in modulation control. Thismay be demonstrated by assuming a direction of vertical deflectioncurrent which produces the magnetic field indicated by the arrows 44.The biasing field 42 and the vertical-deflection-current-producing field44 add in the leg 8 and subtract in the leg 12. The addition of thefields 42 and 44 in the leg 8 tend to saturate that leg and thus disableits control function. The resultant magnetic field in the leg 12,however, is capable of producing a magnetic operating level whichpermits modulation. When the vertical deflection current is in theopposite direction, the arrows 44 will be reversed in direction, andthen the arrow 42 and 44 will add in the leg 12 and subtract in the leg8, thus shifting control from the former to the latter. The degree ofmodulation control will, of course, be determined primarily by theresultant magnetic field in the leg or legs which are exercising controlat any given moment, and that value of magnetic operating level will,for a given magnitude of vertical deflection current, be determined bythe effective magnetic field produced by the biasing magnet 40.

From this analysis it appears that the magnitude of modulation, andhence the magnitude of the resultant pincushion correction, can beachieved by varying the intensity of the magnetic biasing field 42.Further, the pincushion correction at the top and bottom portions of thedisplay (corresponding in FIGS 56 to positive and negative verticaldeflection current respectively) can be varied, so that more correctionis provided at the top than at the bottom or vice versa, by varying thedistribution of the biasing field 42 between the legs 8 and 12. This canbe done by shifting the magnet 40 toward the leg 8 or the leg 12,depending upon the variation desired, the biasing magnetic field thenpartially passing through the control leg 10, so that the biasing fieldsin the legs 8 and 12 are no longer equal. This causes one leg 8 or 12 toproduce a greater modulation control than the other. Since, as we haveseen, one leg predominates for one direction of vertical deflectioncurrent and the other leg dominates for the other direction of thatcurrent, the resultant will be a variation in the amount of correctionproduced at the top half of the display as compared with that producedat the bottom half of the display.

FIGS. 1-3 illustrate a preferred structural embodiment of anelectromagnetic modulator corresponding to that schematically shown inFIG. 7 and capable of effecting the adjustments in modulation effectdescribed above. The magnetic core 6, with the windings thereon, isreceived within a frame generally designated 48 and formed of somesuitable non-magnetic material. It comprises a bottom wall 50 havingoutwardly extending ears 52 with apertures 54 therethrough, throughwhich apertures screws may pass to secure the structure to any desiredsupport. Lips 56 extend up from the side edges of the wall 50, defininga channel within which the magnetic core assembly 6 is relatively snuglyreceived and is constrained against lateral movement. Appropriatelysecured to the bottom wall 50 in any desired manner, and extending uptherefrom between the lips 56, are end walls 58 which substantiallyengage and confine the magnetic core assembly 6 against longitudinalmovement, the walls 58 at each end of the core assembly 6 having straps60 which extend toward one another and are adapted to be securedtogether in any appropriate fashion, as by having the tab 62 on theright hand strap 60 pass through an aperture in the ear 64 on the lefthand strap 60 and then be bent up so as to prevent disengagement of thestraps 60. The magnetic core assembly 6 is constructed as shown in FIG.7, but the individual legs and arms, and the coils carried thereby, areencapsulated or otherwise surrounded by insulating material, theportions 8', and 12 shown in FIG. 2 representing the core legs 8, 10 and12 of the schematic embodiment of FIG. 7 with windings 36, 26 and 38respectively secured thereto and surrounded by encapsulating material.

The end walls 58 of the frame are provided with upward extensions 58awhich project well above the top of the magnetic core 6, and which areprovided with wings 66 which are bent therefrom so as to extend towardthe center line of the core assembly 6. The bottom edges 68 of the wings66 are spaced a short distance above the upper edges 70 of the straps60, the edges 70 more or less corresponding to the upper edges of thecore assembly 6. A carriage generally designated 72 is provided, thatcarriage being generally of H-shape and having a crosspiece 80 fromwhich right and left hand pairs of arms 82 extend, the crosspiece 80resting on the edges 70 and being received between the frame extensions58a with sufficient clearance in the direction from one extension 58a tothe other so that the crosspiece may be shifted in that direction. Thearms 82 lie outside the frame extensions 58a and are received under thewings 66. In this way the wings 66 keep the carriage 72 in verticalposition substantially against the upper edge 70 of the frame straps 60,and consequently substantially against the upper arm 16 of the coreframe 6, While permitting the carriage 72 to be shifted from right toleft as viewed in FIGS. 1 and 2. In order to retain the carriage 72 in agiven adjusted position, a wire spring element 84 is carried by one ofthe frame extensions 58a, and is adapted to resiliently engage aserrated portion 86 on the upper surface of the carriage arms 82.

The carriage 72 carries, in any appropriate fashion, the biasingpermanent magnet 40. As here specifically disclosed the magnet 40 iscylindrical in shape and is magnetically polarized as indicated by theletters N and S in FIG. 1. It is mounted on the carriage 72 so as tomove with the carriage as the latter is moved from right to left inFIGS. 1 and 2, and it is also preferably mounted in the carriage 72 soas to be pivotal about a vertical axis, thereby to move its axis ofmagnetization between positions parallel to and at right angles to thelengthwise dimension of the core arm 16.

As here specifically disclosed, the magnet 40 is received within anaperture 88 in the crosspiece portion of the carriage 72. Partiallysurrounding the magnet 40 are a pair of magnetizable elements generallydesignated 90, each located between the magnet 40 and one of the frameextensions 58a. They are held in position by means of the C-spring 92,have upstanding arcuate shaped portions 94 which partially surround theperiphery of the magnet 40, have top bars 96 which overlie the magnet 40and thereby retain it in position, and have outwardly extending polepiece portions 98 oriented toward the respective frame extensions 58a,the portions 98 being bent down around the left and right hand edges 100of the crosspiece portion 80 of the carriage 72. These elements assistin retaining the magnet 40 in position on the carriage 72, help toconstitute a bearing for the magnet 40, permitting it to be adjustablyrotated in order to vary the effective magnetic biasing force, and byreason of the action of the spring washer 92 they frictionally grasp themagnet 40 and retain it in its rotatably adjusted position. Moreover,the magnetizable portions 98 make a more efficient magnetic circuit forthe biasing magnetic flux, thus intensifying the degree to which thebiasing magnet 40 can be effective in giving rise to the currentmodulations previously described. If desired, special magnetizable polepiece extensions 102 can be provided which extend up vertically insidethe frame extensions 58a from the core legs 8 and 12 respectively topoints substantially on a level with the pole piece portions 98, thusfurther improving the efficiency of the magnetic circuit for the biasingflux.

From the above it will be seen that by rotating the magnet 40 relativeto the carriage 72 the magnitude of the effective biasing magnetic fluxreaching the legs 8 and 12 will be varied, and by shifting the carriage82 to the right or the left, the magnet 40 moving along therewith, therelative distribution of the biasing magnetic force in the legs 8 and 12will be varied, thus providing respectively the adjustment in themagnitude of the sweep current modulation and in the relative magnitudesof that modulation at the upper and lower portions of the scanrespectively.

The structure involved is simple, inexpensive, and need not be made toany high degree of precision, yet through its use a device is producedwhich Will not only give rise to such modulation of the sweep current aswill minimize the pincushion effect in a given direction, but will alsopermit the degree of correction to be adjusted and modified at will.Consequently, a device of a single design can be used with manydifferent cathode ray tube systems and, when used with tubes of a givensystem, can be adjusted to provide corrections appropriate for thedeparture of individual tubes from nominal geometric or electricalcharacteristics. It is, in addition, possible to employ the relativemagnitude correction in applications in which viewing angle problemsmake an unequal pincushion correction of opposing sides advantageous.

While but a single embodiment of the present invention has been herespecifically disclosed, it will be apparent that many variations may bemade therein, all within the scope of the instant invention as definedin the following claims.

I claim;

1. A device for controlling distortion in a cathode ray tube comprisinga magnetic core having first and second legs connected by an arm, abiasing magnet, and means operatively connected to said magnet formounting it adjacent said arm for movement in the direction of thelength of said arm between said legs, and windings on said legs, saidmounting means comprising a frame on which said core is mounted and acarriage mounted on said frame adjacent said core arm and movable in thedirection of the length of said arm, said magnet being mounted on saidcarriage between magnetizable pole pieces which extend from said magnettoward said core legs and are movable with said carriage and magnet.

2. The device of claim 1, in which said core legs have magnetizable polepiece extensions operatively extending therefrom, said magnet and saidmagnetizable pole pieces being received between said pole extensions.

3. In the device of claim 2, resilient means engaging said magnetizablepole pieces and urging them into engagement with said magnet.

4. In the device of claim 1, resilient means engaging said magnetizablepole pieces and urging them into engagement with said magnet.

5. A device for controlling distortion in a cathode ray tube comprisinga magnetic core having first and second outer legs and an intermediateleg all connected by an arm, a biasing magnet, means operativelyconnected to said magnet for mounting it adjacent said arm generally inregistration with said intermediate leg, said magnet being magneticallypolarized substantially in a plane parallel to said arm and beingrotatably mounted in said mounting means to vary the effective magneticstrength in the direction of the length of said arm, windings on saidouter legs and a winding on said intermediate leg, said windings on saidouter legs constituting a control winding, said winding on saidintermediate leg being connected to deflection means in a cathode raytube assembly.

6. The device of claim 5, in which said mounting means comprises a frameon which said core is mounted and a carriage mounted on said frameadjacent said core arm, said magnet being rotatably mounted on saidcarriage.

7. A device for controlling distortion in a cathode ray tube comprisinga magnetic core having first and second legs connected by an arm, abiasing magnet, means operatively connected to said magnet for mountingit adjacent said arm for movement in the direction of the length of saidarm between said legs, said magnet being magnetically polarizedsubstantially in a plane parallel to said arm and being rotatablymounted in said mounting means to vary the effective magnetic strengthin the direction of the length of said arm, and windings on said legs,at least one of said windings being connected to deflection means in acathode ray tube assembly.

8. The device of claim 7, in which said mounting means comprises a frameon which said core is mounted and a carriage mounted on said frameadjacent said core arm and movable in the direction of the length ofsaid arm, said magnet being mounted on said carriage.

9. The device of claim 8, in which said magnet is mounted on saidcarriage between magnetizable pole pieces which extend from said magnettoward said outer core legs and are movable with said carriage andmagnet.

10. The device of claim 8, in which said magnet is mounted on saidcarriage between magnetizable pole pieces which extend from said magnettoward said outer core legs and are movable with said carriage andmagnet,

said outer core legs having magnetizable pole piece extensionsoperatively extending therefrom, said magnet and said magnetizable polepieces being received between said pole extensions.

11. The device of claim 7, in which said mounting means comprises aframe on which said core is mounted and a carriage mounted on said frameadjacent said core arm, said magnet being rotatably mounted on saidcarriage.

12. The device of claim 7, in which said mounting means comprises aframe on which said core is mounted and a carriage mounted on said frameadjacent said core arm and movable in the direction of the length ofsaid arm, said magnet being rotatably mounted on said carriage.

13. The device of claim 12, in which said magnet is mounted on saidcarriage between magnetizable pole pieces which extend from said magnettoward said outer core legs and are movable with said carriage and magnet.

14. The device of claim 12, in which said magnet ismounted on saidcarriage between magnetizable pole pieces which extend from said magnettoward said outer core legs and are movable with said carriage andmagnet, said outer core legs having magnetizable pole piece extensionsoperatively extending therefrom, said magnet and said magnetizable polepieces being received between said pole extensions.

15. The device of claim 7, in which said mounting means comprises aframe on which said core is mounted and a carriage mounted on said frameadjacent said core arm and movable in the direction of the length ofsaid arm, said magnet being rotatably mounted on said carriage, andmeans on said frame operatively engaging said carriage to hold it in anadjusted position along said core arm.

16. A magnetic modulator comprising a magnetic core having first andsecond outer legs and an intermediate leg all connected by an arm, abiasing magnet, means operatively connected to said magnet for mountingit adjacent said arm generally in registration with said intermediateleg for movement in the direction of the length of said arm between saidouter legs, said magnet being magnetically polarized substantially in aplane parallel to said arm and being rotatably mounted in said mountingmeans to vary the effective magnetic strength in the direction of thelength of said arm, and windings on said outer legs and a winding onsaid intermediate leg.

17. The magnetic modulator of claim 16, in which said mounting meanscomprises a frame on which said core is mounted and a carriage mountedon said frame adjacent said core arm and movable in a direction of thelength of said arm, said magnet being rotatably mounted on saidcarriage, magnetizable pole pieces on said carriage between which saidmagnet is mounted and from which magnet said pole pieces extend towardsaid outer core legs, said outer core legs having magnetizable polepiece extensions operatively extending therefrom, said magnet and saidmagnetizable pole pieces being received between said pole pieceextensions, and means on said frame operatively engaging said carriageto hold it in an adjusted position along said core arm.

References Cited UNITED STATES PATENTS 2,702,874 2/1955 Adler 323-92 X2,802,140 8/1957 Mattingly -336 X 3,283,279 11/1966 Garlotte 3361103,346,765 10/1967 Barkow 315-27 RODNEY D. BENNETT, Primary Examiner. R.E. BERGER, Assistant Examiner.

US. Cl. X.R. 336-110

